For the manufacture of convenient dental prostheses, usually, a set of instruments is used in prosthetic dental medicine and in dental technology, including the articulator as a basic apparatus and servicing dental appliances. The articulator is a mechanical instrument, which reproduces temporomandibular joints and to which maxillary and mandibular working models can be fixed, so that some or all mandibular movements can be simulated. Articulators have been systemized into four classes, as the articulators, which allow three-dimensional measurements, belong to the highest class IV. The well-known fully adjustable instruments of this type allow the orientation of working models with regard to temporomandibular joints and the imitation of mandibular movements with sufficient accuracy, but the exact imitation of each one of them cannot be expected, because articulators usually do not have freedom of movement along the three coordinate axes and do not have sufficiently big scope of change along all of them. The main requirement for articulators is they to be able to adapt to the anatomy and physiology of a patient's mouth and not vice versa. It is well-known, that the movements in joint connections are rotating and sliding ones and despite the fact, that the latter are more important ones, the accurate reproduction of all movements is of great importance, moreover, that the mobility of the two temporomandibular joints can be different.
We know an articulator with three relaying degrees of freedom and featuring rotation only in sagittal plane, described in BG 65303B, functioning together with the face-bow and a special tray. The articulator contains an upper plate for the fixation of a maxillary working model and a lower plate for the fixation of a mandibular working model. It contains also a front vertical limiter, which determines the borders of movement at bite, as well a rear frame, maintaining vertical dimension at occlusion, through an analogue of the condylar axis (mandibular transversal hinge axis) of the patient, formed by mechanical analogues of temporomandibular joints. The rear frame and the front vertical limiter connect the upper and the lower plates in a three-dimensional structure. This well-known articulator has the advantage, that it can function not only as a non-adjustable articulator, but also as an adjustable articulator, as a result of which, the number of necessary instruments is reduced and from there and broadly speaking, the consumption of materials at this process. However, the articulator has the demerits, that it does not have the capability for rotation around the three coordinate axes, as well as it cannot be fixed precisely on the axes of articulator joints, whereas mechanical condyles are vertical. This necessitates, at its adjustment, the lower working model, together with the mechanical articulator condyles, before being fixed permanently at more precise individual operations, also to be adjusted additionally, through the face-bow, which action must be repeated at every change of the models for a given patient.
The work with the well-known mechanical articulators and the connected to them face-bow and a bite-fork for adjustment, with respect to individual dental parameters and patient's correlations, is based on the orientation of working models with regard to the fixed in the sagittal plane transversal hinge axis of the adjustable articulator. This principle of articulator adjustment is stipulated in many publications about the way of work with the well-known adjustable articulators, like for example EP 0633006B1, EP 1051952, U.S. Pat. No. 4,315,740A; U.S. Pat. No. 4,460,338A, U.S. Pat. No. 5,385,470A, U.S. Pat. No. 6,287,113, U.S. Pat. No. 6,558,161, BG 65303B1 and others. This method includes the following actions: connection of the impression material of the bite-fork to the upper and/or lower patient's row of teeth; determination of a condylar axis (the transversal hinge axis of a patient) with end-pieces fixed to the face-bow; follows the connection of the face-bow to the handle of the bite-fork or vice versa-first, connection of the face-bow to the handle of the bite-fork and after that, determination of a condylar axis; the taking-out of the face-bow with the bite-fork from the patient's face; mounting of the face-bow with the bite-fork to the articulator and adjustment of the axis between the end-pieces of the face-bow till coincidence with the transversal hinge axis of the articulator; fitting of the upper or lower working model into the impression of the bite-fork; fixation of the working model to the working plate on the relevant articulator frame; dismantling of the face-bow with the bite-fork from the articulator; juxtaposition and adjustment to the fixed working model of an opposite working model; fixation of the opposite working model to the opposite articulator frame; and articulator closure.
In reality, at the application of these methods, there is a practical opportunity for relatively complete spatial orientation of the working model, which first will be fixed into the articulator. However, the demerits of these traditional methods are that, at the adjustment of working models with regard to the transversal hinge axis of the articulator, inaccuracies are possible and undesired displacements of working models one to another, whereas the fixation of working models to the relevant working plate takes place either through material accumulation, which is much more difficult to control and increases the consumption of materials of the models, or through the use of special devices (U.S. Pat. No. 4,315,740; U.S. Pat. No. 4,460,338), which complicates the kinematic chain and the very articulator, without guaranteeing the increase of accuracy and reliability. Besides that, at the repeated dismantling and change of the models in process of work over the prosthesis placement in a certain patient, the articulator adjustment becomes necessary from time to time at every change of the models, which takes time and reduces accuracy. Besides that, the positioning of the models with regard to the real position of the mandibular transversal hinge axis features reduced accuracy, whereas they are adjusted with regard to the transversal hinge axis of the articulator and in this way, there is no reproduction of the specific real position of the mandibular hinge axis of a certain patient over the dental articulator.
Another very important demerit is that at the application of well-known methods, articulator adjustment with respect to the individual dental parameters of a patient is done in a mechanical way, as the face-bow with the bite-fork (tray) is adapted to it, which complicates the whole procedure. Besides that, the connection of the face-bow to the articulator requires a complex multi-hinge connecting element, whose fixation is greatly hampered, as well as an additional supporting element to the articulator base or to the panel onto which the articulator is placed, which reduces the reliability of this connection. Additional inaccuracies and displacements are also possible at the connection of the intra-oral fork (tray) with the face-bow and they together are connected to the articulator.
We know also methods for computer virtual juxtapositions between virtual working models and a virtual articulator, which also are based on the correlations in the model presentation of the jaws in occlusion and an analogue of the condylar axis between temporomandibular joint connections. Virtual working methods include the following actions: scanning of the morphological features of teeth, of rows of teeth and/or of toothless alveolar ridges in the patient's mouth and recording of the patient's individual parameters, construction of virtual working models on their own or in occlusion, the selection of a virtual articulator of an appropriate type, coincidence of the virtual condylar axis of the virtual working models, on their own or in occlusion, with the virtual condylar axis of a virtual articulator, simulation and recording in dynamics of the movements of the mandibular working model with respect to the maxillary working model. The demerits of these methods are that they require costly and complex for work and management specialized CAD/CAM software products at modeling, as well as digital programmable milling machines for the manufacture of prostheses, which increases the service price.
At the application of these computerized methods, virtual articulators also include the upper plate for the fixation of a maxillary working model and the lower plate for the fixation of a mandibular working model. They also contain a front vertical restrictor, which delimitates the movement at the fork-bite, as well as the rear frame, maintaining vertical dimension at occlusion, through an analogue of the patient's condylar axis, formed by analogues of temporomandibular joints. The rear frame and the front vertical limiter connect the upper and the lower plates in a three-dimensional structure.
Experts are looking for ways and means for the reduction of the material consumption of working models, as well as for improving the accuracy of their positioning, with regard to the real position of the transversal hinge axis, which considerably increases accuracy and precision at the manufacture of dental prostheses. I would like also to lay emphasis on the provision of an opportunity for greater adaptability, without requirements for additional skills for work with complex software products.